208 PLANTS, SEEDS, AND CURRENTS 



able for them would be the shortest, namely, from the West African 

 coast to Brazil in the Main Equatorial Current. 



I was familiar with this shore tree in different places, and especially 

 in Jamaica and at Colon; and its pods were a frequent constituent 

 of the drift on the beaches of those localities. The pods were amongst 

 the collection of beach-drift made by Morris in Jamaica, their seeds 

 appearing quite sound (Chall. BoL, IV., 300). When, however, we 

 examine their capacity for trans-oceanic dispersal by currents, we 

 find it insufficient. They are too fragile for withstanding the buffet- 

 ing involved in dispersal by ocean currents for more than a few weeks ; 

 and they soon begin to decay and to admit water, against which the 

 thin coverings of the enclosed seed could afford no protection. The 

 currents have been effective agents in establishing many of the West 

 Indian plants in Bermuda ; but this tree is not one of them. This 

 is very suggestive, because almost all of them are well suited for 

 dispersal by currents ; and the inference is that, though the currents 

 could carry these pods unharmed from island to island in the West 

 Indies, they have not been able to carry them in this condition to 

 the Bermudas. The agency of the drifting log could scarcely be 

 invoked, since, as we shall see, the question is not so much one of 

 buoyancy as of the inability of the pod to preserve the seed from 

 injury through the penetration of sea- water, and this danger would 

 still threaten a pod in the crevice of a drifting log. It is significant 

 that these pods did not come under my notice in the stranded drift 

 of the Turks Islands ; nor does the plant grow in the group. 



The legume is a single-seeded, flat, somewhat oblique, oblong pod, 

 about an inch in length. It floats buoyantly in the dry state; but 

 it owes its buoyancy entirely to the air-bearing tissue in the walls 

 of the pericarp, the seed possessing no floating power and filling 

 the seed-cavity. But for purposes of prolonged oceanic transport 

 the pod is too fragile, and is not sufficiently impervious to water, 

 the thin seed-coverings also offering no protection against sea-water. 

 In appearance the pod does not seem much better fitted for with- 

 standing prolonged sea- water flotation than the dried pod of Pisum 

 sativum. Under the quiet conditions of two experiments made in 

 Jamaica, the pods showed no tendency to sink after a month's 

 flotation, but in some cases water had penetrated and the seed was 

 decaying, and there was little to indicate that the seed would retain 

 its germinative capacity after the pod had been exposed for more 

 than a few weeks to the " rough-and-tumble " of oceanic transport. 



Erythrina 



Reference may first be made to the supposed seeds of this genus 

 thrown up on the Orkney Islands and on the coasts of Scandinavia, 

 which are referred to in Chapter II. (pp. 23, 26). The Orkney drift 

 seed figured by the elder Wallace is evidently an Erythrina seed, and 

 the " Bent-stones " (Buesteen) of the early describers of Scandinavian 

 foreign drift may probably be placed here. Tonning, a pupil of 

 Linnaeus, includes Piscidia erythrina amongst the plants contributing 

 to the Scandinavian drift (Amoenitates Academicce, VII., 477, as quoted 



